Breast cancer is the second leading cause of cancer deaths among women in the U.S. today, claiming 40,000 lives per year. Several factors, including societal influences on childbearing, diet, and environmental chemicals are believed to be impacting the high rate of breast cancer in this country. Reproductive factors are particularly important in the increased incidence in the U.S., two major factors being a decrease in the age of the onset of puberty and the extension of the time of first full-term pregnancy. Reasons for the lower age of puberty are not fully understood, but appear to be due in part to diet, whereas the delay of childrearing is primarily social. Because the development and maturation of the breast is dependent on the time from menarche to pregnancy, there is an extended time in which toxic influences may have an opportunity to induce tumor formation. Our hypothesis is that diet, in particular fatty acid and phytoestrogen composition and quantity, during the perinatal period and childhood determine the level of adiposity in young children. Through leptin and insulin-like growth factor, adiposity determines the pathway of puberty as being driven primarily by the adrenal gland (adrenarche) or ovaries (thelarche), the latter being associated with early menarche and subsequent risk of breast cancer. In two prospective cohorts, from in utero to age 3, and from ages 6 to 12, we shall test the association of diet and adiposity with pubertal pathway, and compare those with a family history of breast cancer, to those without a first or second degree relative with breast cancer. Through analysis of hormones, growth factors, and aromatase activity, we shall examine mechanisms through which diet and familial characteristics may mediate these effects. In rodent studies we shall test the hypothesis that not only does dietary fatty acid and phytoestrogen composition in utero and early life alter puberty and mammary gland maturation, but also alters the periods of life at which mammary glands are most susceptible to carcinogenic insults. Using the power of gene expression arrays we shall define characteristics of initiated mammary epithelial cells that can be used to examine endogenous and exogenous compounds for their carcinogenic potential and better define initiated mammary cells in animal models and human studies.